Melt half of the butter over medium heat in a saute pan and cook the celery and onion until softened, 8 to 10 minutes. Remove from the heat and transfer to a large bowl.
To the bowl, add the mixed vegetables, corn, American cheese and mayonnaise and stir to combine. Transfer the mixture to a square casserole dish and sprinkle the crushed Ritz crackers over top. Melt the remaining butter and drizzle over the top.
Bake for 30 to 35 minutes, or until golden brown on top. Allow to sit for 10 minutes before serving.
Notes
Make Ahead: This can be assembled up to one day in advance. Cover and refrigerate until ready to bake. You may need to add extra baking time if prepared ahead.Nutritional values are based on one serving